Is there any which makes the ATC dialogue more realistic. I'm, sure I've seen some around but when I come to look can't find any.If there are such beasts, which is the best?

you may check for Radar Contact 4, a review is here on Avsim (Gold Star Award http://www.avsim.com/pages/0206/RC4/RC4.htm).

The three add-ons I would consider are Editvoicepack, Radar Contact 4 and VoxATC. Editvoicepack is freeware and modifies the default ATC with several options. Radar Contact 4 is payware and is a very realistic IFR environment. VoxATC allows VFR and IFR flight and uses voice recognition technology so no button pushing. You can find reviews of all of them.

Thanks for that, chaps. I'll check out your suggestions.